A cylinder device and a brake body attached to this cylinder device are equipped as a unit brake for vehicle braking, to activate the cylinder device, and bring a brake shoe retaining relative displaceability versus the brake body into contact with the vehicle wheel, to brake the rotation of the vehicle wheel. For the cylinder device in this type of unit brake, for example, in brake devices for rail vehicles, cylinder devices are known that can be activated with both a regular brake part activated with compressed air (compressed fluid) used in normal operations, and a spring brake part activated by a spring force even without use of compressed air, used for stopping the vehicle over a long time (see Patent Citation 1). In the cylinder device disclosed in Patent Citation 1, the regular brake part has a rod protruding and has installed a first piston used in opposition to a first pressure chamber and first spring, and the spring brake part has a rod penetrating and has installed a second piston used in opposition to a second pressure chamber and second spring. In addition, compressed air is supplied to the first pressure chamber, to move the first piston in the brake direction, in resistance to the first spring added force, and furthermore, compressed air is exhausted from the second pressure chamber, to move the second piston in the brake direction, using the second spring added force.
In addition, in the above-mentioned cylinder device, a clutch function is installed that links the rod and second piston are linked, and releases that link, switching between transmitting or blocking the spring brake part added force. In this clutch mechanism, rotatability is supported versus the second piston, and a nut member screwed into the rod is installed. Moreover, this clutch mechanism is configured so as to move from a state where compressed air is supplied to the second pressure chamber to an exhaust state, and the nut member moves together with the second piston versus the rod, by the second spring added force, so that the rod and second piston are linked to form a linked state. By contrast, in the state where compressed air is supplied to the second pressure chamber, this clutch mechanism is configured so that the link between the rod and second piston is released, to form a non-linked state. In this cylinder device, the clutch mechanism moves to the above-mentioned link state, and is designed so that the convex-concave teeth in the nut member in the clutch mechanism, and in the sleeve member that is the opposite-side member, are linked by mutual gripping, and the brake force is maintained in the spring brake part.